1. Field of the Invention
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) receiver, and more particularly, to an apparatus and method for direct measurement of a channel state using sub-carriers of OFDM signals.
2. Description of the Related Art
CSI (Channel State Information) is generally defined by a SNR (Signal-to-Noise Ratio) of a sub-carrier. There are at least three conventional measurement methods for CSI, an indirect measurement method, a direct measurement method, and a combined method.
A conventional indirect method of channel state measurement uses the magnitude of a channel frequency response calculated in a channel equalizer. This type of method exhibits good performance in a channel with white noise or in a static channel, but does not exhibit good performance in a channel with channel interference, such as a frequency selective channel or a mobile channel. Since analog TV signals are mixed with DVB-T (Terrestrial Digital Video Broadcasting) signals and co-channel interference exists in a co-channel, which has a spectrum as shown in FIG. 7, an indirect method of channel state measurement is not suitable for estimating a channel containing analog TV signals mixed with DVB-T signals.
A conventional direct method of channel state measurement uses differences between a received signal value and the nearest points in an I-Q constellation plot (a constellation plot of In-phase and Quadrature components) as shown in FIG. 5. A description of a conventional direct method is disclosed in a U.S. Pat. No. 5,636,253 or European Pat. No. EP0,991,239. Conventional direct methods exhibit good performance in a channel with channel interference, such as a frequency selective channel or a mobile channel, but do not exhibit good performance in a channel with white noise or in a static channel.
A conventional combined method of channel state measurement discussed in European Pat. No. EP1,221,793, but does not exhibit better performance than a conventional indirect method used for a channel with channel interference, such as a frequency selective channel or a mobile channel.
FIG. 1 is a block diagram of a conventional DVB-T (Terrestrial Digital Video Broadcasting) transmitter. The DVB-T transmitter of FIG. 1 may process an MPEG (Moving Picture Experts Group) bit stream 1 of DVB-T signals and emits the bit stream to the air via an antenna. The DVB-T transmitter may comprise an energy dispersal unit 2, an outer coder 3, an outer interleaver 4, an inner coder 5, an inner interleaver 6, a signal mapper 7, a frame adaptation unit 8, an OFDM (Orthogonal Frequency Division Multiplexing) modulator 9, a DAC (Digital to Analog Converter) 10, and a transmitter front-end 11. As is well known, a signal mapper 7 may generate I (In-phase) and Q (Quadrature) signals according to a modulation format, such as QPSK (Quadrature Phase Shift Keying), 16-QAM (Quadrature Amplitude Modulation), 64-QAM, etc., and a frame adaptation unit 8 changes these signals to a frame structure. Each frame may be made up of 68 OFDM symbols, and each symbol may be made up of a number of active carriers, the number depending on the operation mode. For example, there may be 6817 active carriers in 8k mode or 1705 active carriers in 2k mode. These rules are set as ETSI (European Telecommunication Standard Institute) standards.
A frame adaptation unit 8 may add CPC (Continual Pilot Carrier), SPC (Scattered Pilot Carrier), and TPSC (Transmission Parameter Signaling Carriers) to be used for synchronization, mode detection, and channel estimation to respective OFDM symbols. The positions of these carriers may be set, and as shown in FIG. 2, a scattered pilot insertion pattern may have a form in which every fourth symbol is the same.
FIG. 3 is a block diagram of a conventional DVB-T receiver. The DVB-T receiver of FIG. 3 processes an aerial wave received via an antenna 13 using a procedure inverse to that of the transmitter shown in FIG. 1 and transfers a generated MPEG bit stream to a MPEG processing unit. The DVB-T receiver may comprise a tuner 14, ADC (Analog to Digital Converter) 15, an OFDM de-modulator 16, a sync unit 17, a channel equalizer 18, a TPS decoder 19, a metric calculation and inner de-interleaver 22, a CSI (Channel State Information) processor 24, a Viterbi decoder 25, and an outer de-interleaver, decoder, and de-randomizer 26. The channel equalizer 18 may output an equalized complex OFDM signal and SMCFR (Squared Magnitude of the Channel Frequency Response) and the CSI processor 24 may estimate the degree of certainty for the respective data carriers of an OFDM signal and output CSI.
FIG. 4 is a block diagram of the conventional bit metric calculation and inner de-interleaver 22 shown in FIG. 3. FIG. 4 illustrates a 64-QAM transmission mode. The bit metric calculation and inner de-interleaver 22 may output a symbol, a processing result of CSI, and output signals (I, Q) from the channel equalizer 18 to the Viterbi decoder 25. The bit metric calculation and inner de-interleaver 22 may comprise a symbol de-interleaver 28, bit metric calculators 29-34, bit de-interleavers 35-40, and a bit multiplexer (MUX) 41. A bit metric may be calculated using Equation (1) and the procedure shown in FIG. 5. FIG. 5 is an example of a 16-QAM transmission mode.BMj=CSIk×(|Rk−S0|2−|Rk−S1|2)  Equation (1)where BM is the ith bit metric, Rk is a complex value of a kth carrier, S0 is a value corresponding to a ‘0’ bit at the ith position as a complex value of a nearest point in an I-Q constellation plot, S1 is a value corresponding to a ‘1’ bit at the ith position as a complex value of a nearest point in an I-Q constellation plot, and CSIk is a CSI signal of the kth carrier.
The conventional DVB-T receiver shown in FIG. 3 adopts an indirect measurement of a channel state using SMCFR, and some problems exist, as described above. In order to improve reduce these problems and improve performance in a channel with channel interference, such as a frequency selective channel or a mobile channel, a direct measurement of channel state as shown in FIG. 6 may be implemented. Conventional direct measurement processors are disclosed in Europe Patent Application EP0,991,239 and also shown in FIG.6.
The conventional CSI processor shown in FIG.6 directly calculates CSI from received signals (I, Q) instead of via SMCFR and may output the CSI to the bit metric calculation and inner de-interleaver 22. The CSI processor may comprise a hard quantizer 44, a subtraction unit 45, a modulus circuit unit 46, a data carrier extraction unit 47, a symbol recursive filter 48, and a non-linear circuit 55. The non-linear circuit 55 may further comprise a log calculation circuit 49, an invert circuit 50, a first adder 51, a multiplier 52, and a second adder.
The performance of direct measurement of the channel state may rely on the time it takes for a symbol recursive filter 48 to calculate an average value. In particular, the more OFDM symbols that are used for the averages, the more reliable the measurement of the channel state. However, a problem exists in that this method is suitable for a static channel but not suitable for a dynamic or mobile channel.
For the same reason as shown in FIG. 7, conventional direct methods may not be suitable for DVB-T signal processing with co-channel interference, because a correlation characteristic of each channel is different from each other. Further, as shown in FIG. 8, another problem exists in that conventional direct methods may not suitable for a frequency selective channel (for example, 2K mode, 8K mode, 16K mode, or 32K mode, etc.).